Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter, nitrogen oxides (NOx), and unburned hydrocarbons. Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas. However, to remove diesel particulate matter, typically a diesel particulate filter (DPF) must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common DPF comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system. Excessive backpressure on the engine can degrade engine performance. Generally, particulate matter accumulated on the particulate filter is removed by oxidizing the particulate matter on the filter. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially hazardous situations, accumulated particulate matter is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated. To oxidize the accumulated particulate matter, exhaust temperatures generally must exceed the temperatures typically reached at the filter inlet. Various methods are used to increase the filter inlet exhaust gas temperature and initiate regeneration of a diesel particulate filter.
Conventionally, the filter regeneration event occurs when the exhaust gas temperature is sufficiently increased and substantial amounts of soot are oxidized on the particulate filter. Two types of particulate matter oxidation can occur. First, oxidation can occur in the presence of NO2 at modest filter temperatures (e.g., between about 250° C. and about 400° C.) achieved by modest exhaust gas temperatures to consume modest amounts of particulate matter. Filter regeneration where oxidation occurs in the presence of NO2 is hereinafter referred to as “noxidation regeneration.” Second, oxidation can occur in the presence of oxygen at high filter temperatures (e.g., greater than about 400° C.) achieved by high exhaust gas temperatures to consume large amounts of particulate matter. Filter regeneration where oxidation occurs in the presence of oxygen is hereinafter referred to as “oxidation regeneration.” Unfortunately, the engine exhaust gas temperatures necessary to initiate oxidation regeneration are often sufficiently high to cause safety concerns, premature component failures, and abnormally high wear on the engine.
One engine operating condition of particular relevance to the rate of particulate accumulation on and regeneration of a particulate filter is whether the engine is idling. An engine is idling, i.e., operating in an idle mode, when the engine is not under a load or producing meaningful work, such as engaging a drive shaft to propel a vehicle. Idling typically occurs when the engine is operating within a low engine speed range that is dependent on the type and configuration of the engine. For example, in some conventional diesel internal combustion engines, the engine is idling when operated within an RPM range between approximately 700 RPM and approximately 2,000 RPM, and relatively no work is being performed by the engine.
During extended idling operations of a diesel engine, the particulate filter is prone to filling up with particulate matter at normal idle exhaust gas temperatures. Conventional engine control systems typically wait until a sufficient amount of particulate matter has accumulated on the filter during extended periods of idling operation before commanding a controlled oxidation regeneration of the filter. As discussed above, controlled oxidation regeneration typically consists of driving the filter temperature up to controlled oxidation regeneration temperature levels for a predetermined time period such that oxidation of particulate matter accumulated on the filter takes place. Such large increases in the filter temperature require an equally large increase in the engine exhaust gas temperatures. Because the temperature of exhaust gas generated by the engine when operating in the idle mode is relatively low compared to other engine operating conditions, the temperature swing required to initiate a controlled regeneration event during idle operating conditions is very high. The larger the temperature swing, the less efficient the engine and the more prone the engine is to wear and fatigue.
Based on the foregoing, a need exists for an engine controls strategy that reduces the engine exhaust gas temperature and temperature swing necessary to control particulate matter accumulation on a particulate matter filter during idling operations of the engine.